Calender and embossing bowls are rollers against which material, usually paper, is passed under pressure to impart the desired finish or to ensure uniform thickness. They comprise an elongate steel centre shaft with a flange indented slightly inwards from each end, which flanges define a concentric bowl about the centre shaft. The bowl is filled with a compressed fibre which is usually natural, and mostly cellulosic, but can be synthetic.
Calender bowls are used for "finishing" the surfaces of materials such as magnetic tape, fabrics, or paper, with for example, a high gloss.
Embossing bowls run against engraved steel rollers, defining a rolling nip, each bowl are becoming the female part of an embossing bowl-steel roller pair. Materials such as paper napkins, for instance, are then passed through the nip of the pair.
The bowl filling material used depends on the type of material that is to be "finished", and also on the desired properties t be imparted. Most modern coating mills use woollen paper or cotton paper filled bowls. Thousands of sheets of cotton or woollen paper are axially threaded onto the centre shaft and compressed together under hydraulic pressure to form a compact medium of material. The speeds at which bowls can rotate and the pressures to which the bowl filling material and the material being finished can be subjected, are limiting factors in these processes.
A problem common to these processes, however, is that they require or generate heat. This effect can be detrimental to the contents of the bowl, and with the rise in temperature, the cellulose, or other filling material may overheat and start to burn. The by-products of the combustion of cellulose are carbon and water, the accumulation of which can give rise to pockets of liquid of increased volume within the filling material, principally near the peripheries of the bowls, which in turn give rise to bursts within as well as on the surface of the filling material on the bowl. Thus, bowls which are run under much hotter conditions are traditionally made of asbestos. Although it works well, asbestos is now regarded as an unacceptable material to use on account of the potentially harmful effects to health caused by long-term exposure.
Current efforts at dissipating heat from the bowl material include inserting copper foil discs in between the sheets of woollen or cotton paper which make up the filling. The discs are included to draw heat away from the filling to the centre shaft. In an attempt to further improve the dissipation of heat the centre shaft has been hollowed out and a fluid coolant, usually water, passed through the hollowed shaft. As heat is generated within the bowl, it is conducted along the copper discs to the shaft whereupon the heat is drawn away by the fluid coolant. Few machines can however accommodate such a water cooling process, making the latter approach of somewhat limited application.
It is an object of the present invention to produce a means of dissipating heat from the bowls more efficiently so that heat damage to the bowls can be reduced and running speeds increased.